1. Field of the Invention
This invention relates to fuel elements and assemblies thereof for nuclear thermal engines.
2. Background of the Invention
Nuclear thermal engines utilize fissionable material to heat a propellant, typically hydrogen, which is discharged through a nozzle to generate thrust. A design objective in such engines is to achieve a high thrust-to-weight and specific impulse. This requires a core design featuring a compact configuration with high heat transfer area, a high coefficient of heat transfer and optimum flow rate and flow velocity.
An early design for nuclear thermal engines used prismatic fuel elements in the form of hexagonal rods stacked side by side with a pattern of longitudinal bores in each rod through which propellant flowed axially. Such fuel elements were subject to stress induced cracking between the bores as a result of the very high thermal stresses occurring during operation.
An experimental particle bed fuel element utilizes small fuel beads packed in an annular support. Propellant flows radially inward through a porous peripheral wall, the particle bed and a porous annular inner wall, and then flows axially out through a central passage. These particle bed fuel elements can develop hot spots causing melting of the fuel beads. Thus, while the particle bed fuel elements have a high heat transfer capability, it is difficult to control propellant flow through them and they have poor mechanical stability.
Another proposed design utilizes flat annular plates with radial grooves or holes for propellant flow radially inward to a central discharge passage. However, the very high temperature gradient, for example 100.degree. K at the periphery and 3000.degree. K at the central discharge passage, produces severe stress cracking in the flat, annular plates.
A suggestion has been made to utilize fueled truncated conical shells seated on one another with grooves in the contacting confronting shell surfaces for coolant flow. See, also, U.S. Pat. No. 3,150,054 to Fox, which shows a truncated conical reactor design.
There is a need therefore for improved fuel elements and assemblies for nuclear thermal engines.